This invention in general relates to interferometry and in particular to interferometric apparatus and methods by which reticle and wafer stage slew rates of photolithographic apparatus may be increased while minimizing any deleterious Doppler shift frequency signal processing problems.
Interferometry is a well established metrology used extensively in microfabrication processes to measure and control a host of critical dimensions. It is especially important in manufacturing semiconductors and the like where requirements for precision are 10 to 40% better than critical dimensions of 0.1 xcexcm or below.
Integrated circuits made of semiconductor materials are constructed by successively depositing and patterning layers of different materials on a silicon wafer while it typically resides in a flat exposure plane having Cartesian x-y coordinates to which there is a normal z-direction. The patterning process consists of a combination of exposure and development of photoresist followed by etching and doping of the underlying layers and deposition of another layer. This process results in a complex and, on the scale of microns, very nonhomogeneous material structure on the wafer surface.
Typically, each wafer contains multiple copies of the same pattern called xe2x80x9cfieldsxe2x80x9d arrayed on the wafer in a nominally rectilinear distribution known as the xe2x80x9cgrid.xe2x80x9d Often, but not always, each field corresponds to a single xe2x80x9cchip.xe2x80x9d
The exposure process consists of projecting the image of the next layer pattern onto (and into) the photoresist that has been spun onto the wafer. For the integrated circuit to function properly each successive projected image must be accurately matched to the patterns already on the wafer. The process of determining the position, orientation, and distortion of the patterns already on the wafer, and then placing them in the correct relation to the projected image, is termed xe2x80x9calignment.xe2x80x9d The actual outcome, i.e., how accurately each successive patterned layer is matched to the previous layers, is termed xe2x80x9coverlay.xe2x80x9d
In general, the alignment process requires both translational and rotational positioning of the wafer and/or the projected image as well as some distortion of the image to match the actual shape of the patterns already present. The fact that the wafer and the image need to be positioned correctly to get one pattern on top of the other is obvious. Actual distortion of the image is often needed as well. Other effects, such as thermal and vibration, may require compensation as well.
The net consequence of all this is that the shape of the first-level pattern printed on the wafer is not ideal and all subsequent patterns must, to the extent possible, be adjusted to fit the overall shape of the first-level printed pattern. Different exposure tools have different capabilities to account for these effects, but, in general, the distortions or shape variations that can be accounted for include x and y magnification and skew. These distortions, when combined with translation and rotation, make up the complete set of linear transformations in the plane.
Since the problem is to successively match the projected image to the patterns already on the wafer, and not simply to position the wafer itself, the exposure tool must effectively be able to detect or infer the relative position, orientation, and distortion of both the wafer patterns themselves and the projected image.
It is difficult to directly sense circuit patterns themselves, and therefore, alignment is accomplished by adding fiducial marks or xe2x80x9calignment marksxe2x80x9d to the circuit patterns. These alignment marks can be used to determine the reticle position, orientation, and distortion and/or the projected image position, orientation, and distortion. They can also be printed on the wafer along with the circuit pattern and hence can be used to determine the wafer pattern position, orientation, and distortion. Alignment marks generally consist of one or more clear or opaque lines on the reticle, which then become xe2x80x9ctrenchesxe2x80x9d or xe2x80x9cmesasxe2x80x9d when printed on the wafer. But more complex structures such as gratings, which are simply periodic arrays of trenches and/or mesas, and checkerboard patterns are also used. Alignment marks are usually located either along the edges of xe2x80x9ckerfxe2x80x9d of each field or a few xe2x80x9cmaster marksxe2x80x9d are distributed across the wafer. Although alignment marks are necessary, they are not part of the chip circuitry and therefore, from the chip maker""s point of view, they waste valuable wafer area or xe2x80x9creal estate.xe2x80x9d This drives alignment marks to be as small as possible, and they are often less than a few hundred micrometers on a side.
Alignment sensors are incorporated into the exposure tool to xe2x80x9cseexe2x80x9d alignment marks. Generally there are separate sensors for the wafer, the reticle, and/or the projected image itself. Depending on the overall alignment strategy, these sensors may be entirety separate systems or they may be effectively combined into a single sensor. For example, a sensor that can see the projected image directly would nominally be xe2x80x9cblindxe2x80x9d with respect to wafer marks and hence a separate wafer sensor is required. But a sensor that xe2x80x9clooksxe2x80x9d at the wafer through the reticle alignment marks themselves is essentially performing reticle and wafer alignment simultaneously and hence no separate reticle sensor is necessary. Note that in this case the positions of the alignment marks in the projected image are being inferred from the positions of the reticle alignment marks and a careful calibration of reticle to image positions must have been performed before the alignment step.
Furthermore, as implied above, essentially all exposure tools use sensors that detect the wafer alignment marks optically. That is, the sensors project light at one or more wavelengths onto the wafer and detect the scattering/diffraction from the alignment marks as a function of position in the wafer plane. Many types of alignment sensor are in common use and their optical configurations cover the full spectrum from simple microscopes to heterodyne grating interferometers. Also, since different sensor configurations operate better or worse on given wafer types, most exposure tools carry more than one sensor configuration to allow for good overlay on the widest possible range of wafer types.
The overall job of an alignment sensor is to determine the position of each of a given subset of all the alignment marks on a wafer in a coordinate system fixed with respect to the exposure tool. These position data are then used in either of two generic ways, termed xe2x80x9cglobalxe2x80x9d and xe2x80x9cfield-by-field,xe2x80x9d to perform alignment. In global alignment the marks in only a few fields are located by the alignment sensor(s) and the data are combined in a best-fit sense to determine the optimum alignment of all the fields on the wafer. In field-by-field alignment the data collected from a single field are used to align only that field. Global alignment is usually both faster, because not all the fields on the wafer are located, and less sensitive to noise, because it combines all the data together to find a best overall fit. But, since the results of the best fit are used in a feed-forward or dead reckoning approach, it does rely on the overall optomechanical stability of the exposure tool.
Alignment is generally implemented as a two-step process; that is, a fine alignment step with an accuracy of tens of nanometers follows an initial coarse alignment step with an accuracy of microns, and alignment requires positioning the wafer in all six degrees of freedom: three translation and three rotation. But adjusting the wafer so that it lies in the projected image plane, i.e., leveling and focusing the wafer, which involves one translational degree of freedom (motion along the optic axis, the z-axis or a parallel normal to the x-y wafer orientation) and two rotational degrees of freedom (orienting the plane of the wafer to be parallel to the projected image plane), is generally considered separate from alignment. Only in-plane translation (two degrees of freedom) and rotation about the projection optic axis (one degree of freedom) are commonly meant when referring to alignment. The reason for this separation in nomenclature is the difference in accuracy required. The accuracy required for in-plane translation and rotation generally needs to be on the order of several tens of nanometers or about 20 to 30% of the minimum feature size or critical dimension (CD) to be printed on the wafer. Current state-of-the-art CD values are on the order of several hundred nanometers and thus the required alignment accuracy is less than 100 nm. On the other hand, the accuracy required for out-of-plane translation and rotation is related to the total usable depth of focus of the exposure tool, which is generally close to the CD value. Thus out-of-plane focusing and leveling the wafer require less accuracy than in-plane alignment. Also, the sensors for focusing and leveling are usually completely separate from the xe2x80x9calignment sensorsxe2x80x9d and focusing and leveling do not usually rely on patterns on the wafer. Only the wafer surface or its surrogate needs to be sensed. Nevertheless, this is still a substantial task requiring among other things precise knowledge about the vertical position (the altitude) of the optical projection system above the wafer.
Thus, to achieve precision alignment, interferometric techniques are used for positioning both the reticle (mask) and wafer stages that in general want to operate at different slew rates owing to the differences in scale between the physical dimensions of the reticle and its image as projected onto the wafer. Because there is typically a 4 or 5 to 1 reduction in scale, the reticle stage desirably should be operated at speeds 4 to 5 times faster than that of the wafer and still maintain precision requirements.
The precision with which interferometers can provide such position control has been significantly enhanced by technical advances in the design of various optical components, including lasers, and photosensors. However, the performance of interferometers, and thus the operating slew rates of photolithographic stages is, nevertheless, limited by certain system parameters such as the split frequency between the input reference and measurement beam components available through commonly available laser sources used in dispersion interferometry and heterodyne signal processing applications.
Accordingly, it is a major object of the present invention to provide interferometric apparatus by which the operating slew rates of photolithographic stages may be increased.
It is another object of the present invention to provide interferometric apparatus and apparatus by which the effect of Doppler shifts can be reduced while a high sensitivity to a difference in relative Doppler shifts is maintained when monitoring relative speeds between reticle and wafer stages.
Other objects of the present invention will, in part, be obvious and will, in part, appear hereinafter when reading the following detailed description in conjunction with the drawings.
Interferometric apparatus and methodology for monitoring the relative motion among objects, preferably that of mask and wafer stages in photolithographic processes. The apparatus comprises a plurality of interferometers where each interferometer operates to provide a mixed optical interference signal containing phase information indicative of the motion of a corresponding object. Detector means are provided for receiving the mixed optical interference signals and generating electrical interference signals containing information corresponding to the motion of an associated object. Electronic processing means operate to receive at least one select one of the electrical interference signals and generate a corresponding modified electrical interference signal that is compensated for any Doppler shift differences among the electrical interference signals caused by differences in the relative rates of motion in the objects. Electronic mixing means are provided for receiving the electrical interference signals and the modified electrical interference signal and generating an output electrical interference signal containing information about the relative motion between objects.
In another aspect of the invention each of the plurality of interferometers has a measurement beam that travels over a predetermined optical path and operates to provide a mixed optical interference signal containing phase information indicative of the motion of a corresponding object where the measurement beam of at least one of the interferometers travels over its corresponding predetermined optical path a different number of passes than that traveled by the measurement beams of the other interferometers to compensate for any Doppler shift differences among the mixed optical interference signals caused by differences in the relative rates of motion between the objects.